Method and apparatus for manufacturing an optical fibre cable and cable so manufactured

ABSTRACT

Undesired and unforeseeable shrinkage may occur in a tube of plastic material containing optical fibres following its manufacture, especially during storage when the tube is wound on a reel. As a result, there may be uncontrollable variations of the ratio between length of the tube and length of the optical fibre contained therein (“excess fibre variation”). A method and equipment for limiting the excess fibre variations in a plastic tube, by stretching the tube by a predefined amount during manufacturing are described. The present invention also relates to a plastic tube subjected to a predefined stretching, a cable made of such a tube and the equipment suitable for manufacturing such a tube.

[0001] The present invention relates to a method for manufacturingoptical cables, particularly for manufacturing optical cables containingoptical fibres loosely placed and a cable so manufactured.

[0002] More specifically, one aspect of this invention concerns a methodfor controlling the amount of optical fibre in an elongated jacketsuitable for containing the optical fibre, specifically a tubularelement, typically made of plastic.

[0003] Additional aspects of this invention concern an optical element,consisting of a tubular element containing one or more optical fibres ofcontrolled length, a method for manufacturing this optical element and acable comprising this optical element.

[0004] Currently, the optical fibre manufacturing method consists inloosely inserting one or more optical fibres inside a plastic tube toform the socalled “optical core” of the cable. This element, also knownas “loose tube” or “buffer tube”, can then be used, in differentconfigurations, to manufacture optical cables, singly or in groups ofseveral tubes. These tubes can contain either single optical fibres, orgroups of optical fibres grouped in one or more bundles, or one or moreribbons. Typically, the tubes also contain a filler, e.g. grease, toprevent water from accidentally seeping into the tube and propagatinglongitudinally inside.

[0005] The length of the fibres in the tubes (single, bundles orribbons) can be equal to, longer or shorts than the (axial) length ofthe tube. For the purpose of this description, the difference in lengthbetween fibre and tube will conventionally be called “excess fibre”. Inparticular, when the fibre is longer than the tube containing it, theterm “positive excess fibre” will be used. On the contrary, when thefibre is shorter than the tube containing it, the term “negative excessfibre” will be used. Finally, the term zero excess fibre will be used toindicate that the length of the fibre is substantially the same as thatof the tube containing it.

[0006] Typically, the difference in length of the fibre in the tubeallows cable structure stretching and shrinking caused by, for example,thermal variations or mechanical handling, to avoid cable lengthvariations from affecting the fibre. In fact, unlike polymers, thevitreous material forming the optical fibre is not very sensitive to thetemperature variations that the cable is subjected to during use, but itcan present problems if mechanically stretched. Consequently, the lengthof the fibre in the tube should generally allow the tube to follow thelength variations associated with the stresses (mechanical and thermal)it is subjected to, without imposing undesired mechanical traction orother attenuationcausing phenomena on the fibre. For example, positiveexcess fibre is suitable for high temperature environment or overheadcable optical fibre applications (subject to stretching due to ownweight) to compensate for the structural stretching of the cable inorder to allow the fibre to follow such stretches without sufferingundesired stretches. This ensures that the fibre can follow thestretching without being undesirably stretched itself. On the otherhand, for low temperature environment applications of an optical cable,the structural contraction of such cable tends to increase the excessfibre value. In this case, if a positive excess fibre were used, theadditional increase of the value could cause excessive fibre bending inthe tube, with the risk of inducing signal attenuation. In these cases,the use of negative excess fibre may be suitable.

[0007] Typically in the production of loose optical cores, the plasticmaterial is extruded at high temperature around the fibres to form atube which, once cooled, is wound on special reels.

[0008] One method for making loose cables and controlling excess fibreis described in U.S. Pat. No. 4,414,165 by Oestreich et al. This patentdescribes a method and equipment for forming an optical transmissionelement with loose optical fibres in a tubular jacket containing fillingmaterial.

[0009] Another method for producing loose cables and controlling fibrelength, with respect to the length of the tube containing the fibre, isdescribed in U.S. Pat. No. 5,372,757 by Schneider et al. In particular,as described in this patent, a traction force at high temperature isapplied to the plastic tube and to the optical fibres. The tube is thencooled, maintaining the traction force. The applicant, however, hasobserved that in the lapse of time between tube production andsubsequent application, e.g. to make an optical cable employing thistube, undesired and unforeseeable longitudinal shrinking can occur, withconsequent uncontrollable variations of the ratio between tube lengthand fibre length.

[0010] Consequently, as observed by the applicant, excess fibrevariations must be controlled both during the excess fibre controllingstage on the extrusion line and during the period from production of thetube, which is typically wound on a reel at the end of the productionprocess, to its subsequent employment for making the cable. Typically,storage times (i.e. the time in which the tubes are wound on the reelbefore being used to make the cable) vary from several hours toapproximately one week.

[0011] In particular, the applicant has observed that once the opticalcores, made according to known techniques, are collected on- a reel, theplastic material forming the tube tends to additionally settle and, inparticular, shrink. This settling generally cannot be foreseen; however,it usually causes additional tube shrinking leading to uncontrollablevariations—usually increases—of the set excess fibre values.

[0012] The shrinking observed by the applicant in some cases results insizes comparable to the excess fibre value set in production, with theresult of substantially modifying the final excess fibre value andcreating problems in the subsequent use of the tube in making theoptical cables.

[0013] In particular, the applicant has observed that, at highproduction speeds, the tube is typically wound on the reel in randomcrossed turns. This unorderly tube winding generates gaps randomlydistributed on the tube skein collected on the reel. The tube maydetensionate more easily near these gaps and shrink, while detensioningmay be obstructed in other areas. This causes different, uncontrolledshrinking of the tubes wound on different reels and also along differentlengths of the same tube wound on the same reel.

[0014] Having defined the problem, the applicant has found a solution toeliminate, or at least minimize, these length variations during thestorage of plastic tubes containing optical fibres, by stretching thematerial forming the tube containing the optical fibres by a predefinedamount.

[0015] One aspect of this invention, therefore, relates to a method forproducing polymeric material tubes associated with one or more opticalfibres comprising the following steps:

[0016] feeding at least one optical fibre along a path to an extruder;

[0017] extruding the polymeric material around said optical fibre toform the tube;

[0018] cooling the tube to a predefined final temperature; the followingsteps are performed during cooling:

[0019] applying a first traction force to the tube containing saidoptical fibre in a first section of said extrusion line;

[0020] applying a second traction force to said tube in a second sectionof said extrusion line, in substantial absence of congruence betweensaid fibre and said tube since said second traction force is greaterthan said first traction force;

[0021] applying a third traction force to said tube in a third sectionof said extrusion line, said third traction force being less than saidsecond traction force;

[0022] said second traction force will reduce tube longitudinalshrinking by at least 20% after a storage period of one week or longerimmediately after extrusion, compared to a similar tube which is notstretched.

[0023] Preferably, such second traction force is applied at a tubetemperature when the modulus of elasticity of the polymeric material isapproximately 2000 Mpa, preferably between approximately 100 Mpa andapproximately 2000 Mpa, or more preferably between approximately 300 Mpaand approximately 1500 Mpa.

[0024] Preferably, said final temperature is lower than approximately40° C., preferably approximately 20° C.

[0025] The tube temperature variation during the application of thesecond traction force is limited.

[0026] Preferably, the temperature variation in the tube lengthsubjected to the traction force is approximately 10% lower than thetotal thermal gap subjected by the tube along the extrusion line;preferably, the temperature variation in the tube length subjected tosaid second traction force is lower than approximately 20° C. and morepreferably lower than approximately 10° C.

[0027] According to a preferred embodiment, said second traction forceis predefined to induce a stretching of approximately 1% or more whenthe polimeric materials of the tube is polybutyleneterephthalate (PBT).

[0028] A second aspect of this invention relates to a polymer tubeproduced by extrusion comprising one or more optical fibres, allocatedinside said tube, characterized by the fact that, during production,said tube is subjected to stretching so that its longitudinal shrinkageis 20% or more less than that of a tube which was not stretched, after astorage period of one week or longer immediately following extrusion.

[0029] Preferably, said tube should be made of polybutyleneterephthalate(PBT), polyethylene (PE) or polypropylene (PP) polymeric material.

[0030] Preferably, such stretching is approximately 1% or more for PBTpolymeric material tubes.

[0031] In a further aspect, this invention relates to equipment formaking a tube containing one or more optical fibres comprising:

[0032] an extruder suitable for producing a plastic material tubecontaining one or more optical fibres;

[0033] one or more cooling pools;

[0034] a stretching device suitable for applying traction to a length ofsaid tube, with temperature variations in said tube length 10% lowerthan the total thermal head of the tube from extruder to ambienttemperature.

[0035] In particular, this stretching device comprises a driving elementand a braking element, located between the extruder and said drivingelement.

[0036] Said driving element can comprise a drive wheel or a pair ofdrive tracks. The braking element can comprise, in turn, a second drivewheel or pair of drive tracks where the tube is fed at a lower speedwith respect to the speed of the tube at the driving element.Alternatively, such braking element can either be an idle wheel aroundwhich the tube is wound by one complete turn and to which a brakingforce is applied, or an inflatable sleeve with a substantially circularcentral opening in which the tube slides.

[0037] Preferably, said stretching device comprise a first drive wheel,set at a first revolution speed, and a second wheel, set at a slowerrevolution speed than the first.

[0038] Alternatively, said stretching device comprise:

[0039] a first drive device, suitable for stretching said tube at afirst speed;

[0040] a second driving element, set at a speed substantially equal tothe speed of said first drive device;

[0041] a third element, located between said two driving elements,suitable for applying a force directly perpendicular to said tubefeeding direction on the length of the tube between the two drivingelements.

[0042] Alternatively, the stretching device comprise:

[0043] a driving element;

[0044] a braking element, comprising two set of rollers between whichthe tube is fed; being such two set of rollers arranged alternatively atopposite ends with respect to the central tube axis so that the distancebetween the lower surface tangent of the upper set and the upper surfacetangent of the lower set is smaller than the diameter of the tube by acertain value to cause predefined stretching of the tube.

[0045] The present invention will be better explained by the followingdetailed descriptions, with reference to the accompanying figures,where:

[0046]FIG. 1 shows a schematic example of a state of the art extrusionline;

[0047]FIG. 2 shows a schematic example of a extrusion line with thefirst example of equipment according to the present invention;

[0048]FIG. 3 shows a schematic example of an extrusion line with asecond example of equipment according to the present invention;

[0049]FIG. 4 shows a schematic example of an extrusion line with a thirdexample of equipment according to the present invention;

[0050]FIG. 5 shows a schematic example of equipment according to thepresent invention;

[0051]FIG. 6 schematically illustrates a first example of a drivingelement suitable for imposing a predefined speed on the tube in theaforesaid equipment in order to achieve the desired tube stretching;

[0052]FIG. 7 schematically illustrates a first example of a drivingelement suitable for imposing a predefined speed on the tube in theaforesaid equipment so as to achieve the desired tube stretching,comprising a track to prevent the tube from slipping;

[0053]FIG. 8 schematically illustrates the front view of a first exampleof a braking element which can be used to achieve the desired tubestretching, according to the present invention;

[0054]FIG. 9 schematically illustrates the side view of the same exampleas depicted in FIG. 8, according to the present invention;

[0055]FIG. 10 schematically illustrates the front view of a secondexample of a braking element which can be used to achieve the desiredtube stretching, according to the present invention;

[0056]FIG. 11 schematically illustrates an example of a device with twodrive wheels to produce tube stretching;

[0057]FIG. 12 shows the bottom view of the same device as depicted inFIG. 16;

[0058]FIG. 13 schematically illustrates an example of a buffer;

[0059]FIG. 14 schematically illustrates a first example of a cablecomprising at least one loose optical fibre plastic tube, according tothe present invention;

[0060]FIG. 15 schematically illustrates a second example of a cablecomprising at least one loose optical fibre plastic tube, according tothe present invention;

[0061]FIG. 16 schematically illustrates a third example of a cablecomprising at least one loose optical fibre ribbon plastic tube,according to the present invention; and FIG. 17 shows an example of howcan vary the traction applied to the tube, the tube temperature and theexcess fibre value in the various extrusion line sections, according toa method of the present invention.

[0062] Typically, to make a plastic material tube containing inside oneor more fibres, such tube is extruded around the optical fibres.

[0063] As shown schematically in FIG. 1, a conventional extrusion linefor manufacturing a tube containing at least one optical fibre Itypically comprises at least one reel from which one or more opticalfibres I are taken and sent to an extruder head 3, through which theplastic material is extruded around them, forming a tube 11. The tube 11is then sent to a cooling device 10, and from there to a stretchingdevice 5 (typically with a diameter of 600 mm to 1000 mm) and then to afinal collection reel 4. Optionally, the extrusion line can comprise anadditional pulley 7 (also with a typical diameter of 600 mm to 1000 mm)arranged between the stretching element 5 and the extruder.

[0064] The fibres and the plastic material tube enclosing them proceedfor a certain length along the extrusion line, each independently fromthe others. The cooling, and consequent shrinking, of the plasticmaterial does not cause excess fibre value variations in this section,as the fibres are not integral with the tube and shrinking isdistributed along all the tube part not integrally bound to the fibres.

[0065] In order to generate or modify excess fibre, on the other hand,fibres and tube must proceed integrally with respect to each other alongthe extrusion line, so that the longitudinal shrinking of the plastictube caused by cooling, generates the desired fibre excess, due toessentially no or considerably less fibre shrinking. The point on theextrusion line where such integral movement of fibre and plastic tube isachieved is called “congruence point” and from this point onwards fibreand tube are defined as “congruent”.

[0066] Congruence is typically caused by reaching a friction thresholdbetween optical fibres and tube, generally favoured by tube coolingalong the extrusion line, and by the possible presence of filler insidethe tube.

[0067] Under equal process conditions, the congruence point can beshifted along the extrusion line by suitably adjusting the tube coolingarrangement. For example, the congruence point can be moved closer tothe extruder by decreasing the temperature in the cooling pool.

[0068] Alternatively, congruence between fibre and tube can bemechanically forced at a certain point of the extrusion line. Forexample, a wheel can be used (e.g. the pulley 7 in FIG. 1) where thetube containing the fibre is wound by a certain number of turns (e.g.two or more) in order to increase friction and prevent the optical fibrefrom slipping inside the tube. In this way, the desired congruencebetween fibre and tube is created at the wheel.

[0069] Essentially, excess fibre depends on the temperature at whichtube and fibre congruence occurs. A higher plastic material temperatureat the congruence point will lead to greater shrinking downstream withrespect to this point and consequent higher excess fibre generated byshrinking.

[0070] So, in an initial approximation:

[0071] ΔL/L=a(T)ΔT

[0072] ΔL tube sample stretching

[0073] L tube sample length

[0074] a(T) thermal dilation coefficient (according to temperature)

[0075] ΔT temperature variation between congruence point and end ofcooling transient.

[0076] Accurate evaluation of the phenomena shall also consider othervariables, such as axial stiffness, exchanged traction, fillerviscosity, etc., which depends on temperature.

[0077] Accordingly, having defined the process conditions and given thecharacteristics of the plastic material, especially the thermal dilationcoefficient, the fibre-tube congruence can be attained at optimaltemperature so as to achieve the desired fibre excess.

[0078] Typically, at the end of the production process, the final excessfibre in the optical tube is equal to several tenths of a percentagepoint and, in particular, varies from approximately −3⁰/_(oo) toapproximately +3⁰/_(oo), preferably from approximately −1⁰/_(oo) toapproximately +1⁰/_(oo).

[0079] Controlled tube traction according to this invention is seteither by inserting devices along the extrusion line to impose differentspeeds on two lengths of said tube in a controlled fashion, or byapplying a plastic stretching force to the tube. In particular, thetraction is higher than that usually applied to tubes under normalextrusion conditions, i.e. preferably 2 to 5 times the traction normallyapplied to the tube.

[0080] According to a first example, shown schematically in FIG. 2, thetube 11 from the extruder 15 is subjected to the desired traction forceby a device which comprises a first driving element 12 (e.g. a drivepulley with a predefined revolution speed around which the tube is woundfor one or more complete turns without slipping) and a second drivingelement 13 with braking functions, located upstream of the drivingelement 12 (e.g. a second pulley around which the tube is wound for oneor more complete turns without slipping, set at a lower revolution speedthan that of the pulley 12). The device is located at a distance L1 fromthe extruder so that the tube reaches the desired traction applicationtemperature. Such distance also depends on the material type, the tubeextrusion speed and coolant temperature. For example, such distance willincrease as the extrusion speed and coolant temperature increase.Typically, this distance will be between 1 m and 10 m.

[0081] The distance L2 between the two driving elements 12 and 13 willbe sufficiently reduced so that the length of tube between the twoelements is subjected to limited temperature variations. In principle,such distance is less than one meter, preferably between approximately200 mm and approximately 500 mm, more preferably between 300 mm and 400mm.

[0082] The tube temperature is taken to desired values for tubestretching by means of one or more cooling pools 10 located along theextrusion line.

[0083] In a preferred configuration, several cooling pools 10 a, 10 b,10 c aligned in sequence are used. The overall longitudinal length L3 ofcooling pools should preferably be between 10 m and 50 m and suchcooling pools could be filled with coolant at various temperatures.

[0084] A particularly preferred configuration foresee the stretchingdevice, comprising two driving elements 12 and 13, should be containedinside a cooling pool 10 b.

[0085] The tube advances at different speeds on the two driving elementsto cause stretching in the length comprised between the driving elementsas expressed by the formula:

ε=(L _(a11) −L ₀)/L ₀=(V _(A) −V ₀)/V ₀=(V _(A) /V ₀)−1

[0086] where L₀ is the initial tube length, L_(a11) is the length oftube subjected to traction, V_(A) and V₀. are the tube speeds on drivingelements 12 and 13, respectively.

[0087] A predefined traction on the length of tube between the twodriving elements 12 and 13 is applies by suitably setting the revolutionspeed of the two driving elements 12 and 13 for stretching.

[0088] According to this invention, the traction applied to such lengthof tube is higher than the traction applied to the tube in otherextrusion line sections, in particular in the section following the linesection where the stretching device described above is located.

[0089] The traction applied to the tube in this section will preferablybe 2 to 5 times the traction normally applied in the following extrusionline section, in particular between the element 12 and the drive wheel5.

[0090] For example, if the tube is stretched by a traction force ofapproximately 1 kg in the section between the pulley 12 and the drivewheel 5, such tube can be advantageously subjected to a traction ofapproximately 2.5-3 kg in the section between the pulleys 12 and 13.

[0091] Stretching of the plastic material of the tube is performedbefore congruence is achieved between tube and fibre. In this way,during tube stretching, the fibres move independently with respect tothe tube and consequently are not subjected to the stretching tractionstress imposed on the tube.

[0092] The optical fibres can generally resist stretching of up to 0.3%without being damaged. Once this threshold is exceeded, stresses may becreated in the fibre which generally cause attenuation of thetransmitted signal.

[0093] Without congruence, as a result, the tube can be stretched by arelatively high amount, e.g. in the order of 1% or more, withoutsubjecting the optical fibres to undesired stress.

[0094] Regardless of the type of device used to apply the traction forceon the tube, the length of tube subjected to traction shouldnevertheless be limited so that the stretched length of tube issubjected to limited temperature variations.

[0095] If the tube temperature variation is limited, the polymer modulusof elasticity is also subjected to limited variations, thus allowingbetter process condition controlling.

[0096] Preferably, the temperature variation in the length of tubesubjected to traction should be approximately 10% less than the totalthermal gap of the tube along the extrusion line.

[0097] For example, for PBT (polybutyleneterephthalate) polymericmaterial, which has an extrusion temperature of approximately 300° C.,the thermal gap to reach the ambient temperature of 20° C. isapproximately 280° C.; an acceptable temperature variation in the lengthof tube subjected to stretching will accordingly be approximately 28° C.

[0098] To further limit variability of the material's modulus ofelasticity, the temperature variation in the length of tube subjected tocontrolled traction should be less than approximately 20° C., preferablyless than approximately 10° C.

[0099] To limit aforesaid temperature variation, the length of tubesubjected to controlled traction should be under one meter, preferablybetween approximately 200 mm and approximately 500 mm.

[0100]FIG. 17 shows a schematic example of the traction, temperature andexcess fibre patterns in a tube along the various extrusion linesections shown in FIG. 2. The numeric values of this example arespecifically referred to extruding a PBT tube with an internal diameterof 2 mm, external diameter of 3 mm and containing 6 optical fibres witha diameter of 250 μm.

[0101] In section S1 (approximately 3 meters long) comprised between theextruder 3 and the wheel 13 of the stretching device, the traction(lower graph) is required to make the tube and fibres proceed along theextrusion line (0.2 kg). In this first section, the tube temperature(middle graph) decreases exponentially by approximately 300° C. at theextruder outlet to approximately 60° C. At this temperature thepolymeric material has the desired modulus of elasticity. With nocongruence between fibre and tube, the excess fibre value is zero (uppergraph).

[0102] In section S2 between the two wheels 13 and 12 of the stretchingdevice a traction force of approximately 2.5 kg is applied to the tubeat a deformation speed of approximately 0.6 m/min. The length of tube towhich the traction is applied is shown in the diagram, for the sake ofsimplicity, as the length of the tube between the axes of the twopulleys 12 and 13 (approximately 0.5 m).

[0103] ln this section, the temperature variation is contained within10° C., so as to minimize the variations of the value of the polymericmaterial's modulus of elasticity. There is no congruence between thefibres and tube in the section between the two pulleys so the stretchinggiven to the tube is not transmitted to the fibres. Instead congruencebetween tube and fibres is created by wheel (12) so that the fibresproceed integrally with the tube at the outlet of section S2.

[0104] In section S3, the traction applied to the tube is taken to thevalues normally applied in extrusion lines, in this case approximately0.8 kg. In this section, the congruence between fibre and tube and theadditional temperature decrease (from approximately 60° C. toapproximately 20° C.) generate the desired fibre excess (approximately1%) in the tube by the effect of the thermal shrinking of the polymericmaterial.

[0105]FIG. 7 schematically shows an example of driving element 12 andbraking element 13 (according to the configuration shown in FIG. 2).Such element consists of a drive wheel and possible additional apparatus24 to prevent tube 11 from slipping on said drive wheel.

[0106] Said apparatus 24 consists of three idle wheels 23 arranged toform a triangle, between which a belt 19 slides. The apparatus 24 islocated so that two of the three idle wheels 23 are disposed over onepart of the drive wheel.

[0107] The belt 19 is pulled to hold the tube against the drive wheeland prevent undesired slipping of the tube. Belt tension and the lengthof the belt in contact with the tube 11 are adjusted according to, forexample, drive wheel position, tube temperature and tube material. Saidpart of the belt in contact with the tube is varied by positioning thetwo idle wheels 23 over the drive wheel so that the belt takes the shapeof the wheel. The greater the part of the drive wheel between the twoidle wheels, the greater the part of the tube adhering to the belt 19.

[0108] If the drive wheel is also used to produce congruence betweenfibre and tube in the extrusion line, advantageously the tube 11 iswound around the drive wheel a certain number of times to make the tubemovement integral with that of the fibres it contains. FIG. 12 shows anexample of multiple wrapping (four turns) around a drive wheel. In thiscase, a three-wheel device with a belt suitable for preventing tubeslipping may not be required.

[0109]FIG. 6 shows an alternative driving element configuration forpulling the tube along the extrusion line comprising a pair of drivetracks.

[0110] The tube 11 is pulled by the specific drive tracks, essentiallyin a linear fashion, by two sets of drive wheels 21 located on the twoopposite sides of the tube. The number of drive wheels 21 of the track(six in the figure) also depends on tube material and on position on theextrusion line, so as to prevent slipping between the polymeric materialof the tube and the drive wheels 21 and to avoid undesired and harmfulvariations of the rated feeding speed of the tube without, however,exerting excessive pressure on the tube. Such driving element can beused instead of one or both drive wheels as shown above, preferablyinstead of the braking element.

[0111] According to an alternative solution, stretching of the tubecoming from the extruder is performed by imposing a force directlyperpendicular to the direct traction force applied by a first drivingelement and a second driving element located along the extrusion lineand suitable for applying tension to the part of the tube subjected tosuch force perpendicular to the direct traction force.

[0112] For example, as shown schematically in FIG. 4, such forceperpendicular to the direct traction force going from a driving elements13 to a driving elements 12 located along the extrusion line is producedby applying a force (e.g. a weight, a spring or similar) to the tubeusing a wheel 17 to which a suitably calibrated weight 18 is connected.In this configuration, the driving element 12 makes the tube and fibrescongruent.

[0113] According to a different configuration example, illustratedschematically in FIG. 3, the stretching of tube 11 coming from theextruder 15 can be performed by imposing a traction force T on thelength of tube in a controlled fashion, by means of a stretching drivingelement 12 and a suitable braking device 16. This is expressed by thefollowing formula:

ε=T/EA

[0114] where EA is the tube axial stiffness.

[0115] The tube temperature is taken to the desired values forstretching by means of one or more cooling pools 10 located along theextrusion line. In one configuration example, shown by a dotted line inFIG. 3, the braking element 16 and the stretching driving element 12 arecontained inside a cooling pool 10.

[0116] According to one configuration example, the braking element 16consists of sliding shoes and elastic, pneumatic, hydraulic tighteningdevices or similar.

[0117] According to another configuration example, the braking element16 consists of an idle wheel to which a brake is applied, e.g. afriction brake, around which the tube is wound forming a predefinedarch. Preferably, such wheel also comprises a system for preventing tubeslipping, e.g. a track with three wheels positioned to form a triangle,as shown schematically in FIG. 7.

[0118] According to another configuration example, shown schematicallyin FIGS. 8 and 9, the braking element 16 consists of a sliding sleeve 38made of elastic material, with an outer side 46 and an inner side 47into which the tube 11 containing the optical fibres 1 is fed. Thelength of the sleeve is mainly defined according to the tube materialand its temperature in the position where the braking element 16 islocated. This inflatable sleeve 38 is equipped with a fitting 37attached to an inflation system. The sleeve 38 is inflated by thefitting 37 and taken to a pressure so that the sides of the sleeve 38 incontact with the tube 11 exert a uniform pressure along the entirelength of the sleeve 38, regardless of possible variations of thedimension of the extruded tube, so as to brake the tube 11 by frictionwithout damaging or deforming it.

[0119] Such braking device allows to reduce to the minimum any undesiredtube ovalization occurring when the plastic material is subjected tostretching under excessively high temperature conditions in the presenceof unequal stress.

[0120] In another configuration, shown schematically in FIG. 10, theinflatable sleeve 38 made of elastic material is surrounded by a stiffcasing 40 to prevent dilation of the sleeve 38 in the opposite directionof the tube 11 subsequent to internal pressure of the sleeve 38. Thestiff casing 40 ensures that the dilation induced on the inflatablesleeve 38 causes sleeve dilations only in the direction towards the tube11.

[0121] Tube stretching can also be produced by coupling a device, suchas the one shown schematically in FIG. 5, to a drive wheel 12 betweenthe wheel and the extruder. Such device comprises two sets of displacedaxis rollers between which the tube passes. Such two sets of rollers 21a and 21 b are arranged alternatively at opposite ends with respect tothe central tube axis so that the distance between the lower surfacetangent of the upper set 21 a and the upper surface tangent of the lowerset 21 b is less than the diameter of the tube. The deviation from thelinear path accordingly imparted to the tube, in the part of the tubesubjected to the greatest bending, causes micro-stretches which, addedtogether, provides the desired predefined stretching. Such device inthis configuration example is preferable for tubes with a very lowmodulus of elasticity, e.g. less than 200 Mpa.

[0122] The devices mentioned above for stretching the tubes can bemobile and suitably positioned along the extrusion line according to thevarious requirements, especially to carry out the stretching process atthe most convenient temperature.

[0123] As mentioned above, the congruence between optical fibres andtube can be set at any point of the extrusion line after the stretchingdevice, using a device consisting of wheels or drums, either idle or,preferably, driven, upon which to wind the tube a certain number oftimes so as to prevent additional slipping between fibre and tube.

[0124] Preferably, congruence is achieved at a driving element thatcoincide with the driving element 12 shown in FIGS. 2, 3 or 4. Thisensures the simplest configuration as the tube stretching device alsodefines fibre excess.

[0125] The excess fibre value can also be controlled during the tubeproduction process, e.g. by comparing revolutions of a wheel locatednear the fibre feed reel (upstream with respect to the extruder)-turningintegrally (without slipping) with the fibre-to the revolutions of awheel revolving without tube slipping at ambient temperature (i.e. whenthe thermal shrinking is basically finished), e.g. near the collectiondrum.

[0126] With reference to FIGS. 2, 3 and 4, suitable traction controllingdevices 14, suitably located along the length of tube subjected tostretching, are employed to effectively control tube traction.

[0127] The traction applied can be effectively controlled by directmeasurement, e.g. by means of a “buffer” located along the stretchinglength or by means of one or more “load cells” located in the braking ordriving equipment. The term “buffer” refers to a system, shownschematically in FIG. 13, typically consisting of two freely turningpulleys 42, 43, one of which (42) has a fixed axis of revolution and theother (43) a mobile axis with respect to the axis of pulley 42,preferably fitted either on guides or on a rocking rod 44 and suitablycounterbalanced with a counterweight 45 so as to balance the tension ofthe tube 11 and the pressure exerted by the mobile pulley 43. Once apredefined traction is set on the tube, traction variations will beabsorbed by the relative movement of the mobile pulley with respect tothe fixed pulley, keeping the stretching traction unchanged.

[0128] The term “load cells” is used to describe a device comprising afreely turning pulley in partnership with a pressure sensor, e.g. adevice marketed with the trademark Tension transducer ATB 05 made by ASAAutomazione Torino.

[0129] Alternatively, the speeds V₀ and V_(A) at the two ends of thelength of tube being stretched can be measured, i.e. at the beginningand at the end of the tube length between the driving element 12 and theelement is 13 or 16, from which tube stretching can be computedaccording to the formula expressed above.

[0130] These controlling devices act in order to keep the pressureexerted on the plastic material as constant as possible, interveningretroactively on the system after detecting variations of tubestretching or speed V₀ and/or V_(A), by suitably increasing ordecreasing the revolution speed and/or the braking intensity on theelements defining tube stretching. Preferably, these retroactive actionsare controlled electronically.

[0131] According to a preferred embodiment, it appears particularlyconvenient to stretch the polymeric material in an area of the extrusionline where the tube has been adequately cooled so as to prevent risks ofovalization, but still having a temperature suitably high so the modulusof elasticity of the tube is still sufficiently low in order to favourstretching of the tube. The temperature at which stretching is performedwill ensure that the modulus of elasticity of the tube material,according to this invention, is preferably between approximately 100 Mpaand approximately 2000 Mpa, more preferably between approximately 300Mpa and approximately 1500 Mpa. For materials generally used to makesaid tubes, such temperature is generally between 20° C. and 100° C.,preferably between 30° C. and 70° C.

[0132] The applicant has observed that with the method of the presentinvention, the longitudinal shrinking of the tube after production canbe considerably limited, with special reference to storage. Inparticular, for a tube subjected to controlled traction according tothis invention, the tube shrinking can be substantially reduced withrespect to similar tubes which did not undergo a similar traction,during the aforesaid storage. More precisely, the applicant has observedthat the shrinkage of a tube produced according to this method is atleast 20% less than that of a similar tube made according to traditionalprocesses.

[0133] The amount of stretching to which the tube is subjectedconsequently must be sufficiently high in order to ensure a certainreduction of shrinkage during storage; however, the stretching of theplastic material must be suitably limited so as such stretching not toexcessively deteriorate the mechanical properties of the tube.

[0134] In addition to the traction value, the speed with which tractionis applied also appears important. With reference to FIG. 17, thestretching traction application speed is particularly defined by thedifferent tube speed in section S3 and in section S1. Typically, suchapplication speed is between approximately 0.1 m/min and approximately 2m/min.

[0135] For example, in practical observations, a PBT tube subjected tostretching by approximately 10% at a deformation speed of approximately0.6 m/min presents the desired shrinking reduction characteristicsduring storage.

[0136] Materials used advantageously for making optical fibre tubescomprise polyalkyleneterephthalates and polyolefins, in particularpolybutyleneterephthalate (PBT), polyethylene (PE) and polypropylene(PP).

[0137] Although the results of this invention should not be limited tospecific theories, the applicant believes that a possible reason for thereduction in longitudinal tube shrinkage after production should lie inthe fact that tube stretching on the extrusion line, given theconditions in which this is performed, exceeds the elastic stretchinglimit value of the tube. Exceeding such elastic stretching limit (oryield point) in the tube, or in a part of its length, causes permanentdeformation, which could be the cause of the reduction in observedshrinkage in the tube during the stages following production.

[0138] An example of an optical fibre cable made according to thisinvention is shown in FIG. 14. The cable in FIG. 14 presents, in theinnermost radial position, a central member 32, made typically of fibreglass, coated with a polymer jacket 28, e.g. polyethylene. The cablepresents one or more PE, PBT or PP tubes 26, which may be embedded in afiller 33, loosely containing optical fibres 25, also embedded in afiller 33. The filler 33, if required, can be replaced withwater-swellable powder or other water-blocking material. A predefinedstretching was applied to tubes 26, according to this invention. Thetubes 26 are coated with a reinforcement layer 29, typically made ofKevlar® or glass fibre, comprising two sheath cutting strands 34,located longitudinally with respect to the cable. Finally, the cablecomprises a corrugated metal tape 30 (if required) and an externaljacket 27, typically made of polyethylene.

[0139] Another example according to the present invention is shown inFIG. 15. It consists of a cable with single tube 26 loosely containingthe optical fibres 25. In particular, FIG. 15 shows a section view of afibrep optic cable which comprises a stretched central tube 26,according to this invention, containing the loose optical fibres 25embedded in the filler 33, if required. The radius of the central tube26 is surrounded by a reinforcement layer 29, comprising two sheathcutting strands 34, a corrugated tape 30 and an external polyethylenejacket 27.

[0140] An additional cable example is shown in FIG. 16. The structure ofthis cable is similar to that described in FIG. 15. The difference isthat the optical fibres 25, loosely contained in the suitably stretchedtube 26, are grouped in ribbons 31. Furthermore, two dielectricreinforcement elements 35, e.g. made of fibre glass, are next to thesheath cutting strands 34.

[0141] The insertion of additional components in the cable describedabove can be performed, according to known techniques, from the tubemade according to this invention and for this reason are not describedin greater detail.

[0142] The present invention is illustrated in greater detail in thefollowing practical example.

EXAMPLE

[0143] A Vestodur 3000 polybutyleneterephthalate loose optical fibretube was made using the equipment described in FIG. 2. Such tube had aninternal diameter of approximately 2 mm and an external diameter ofapproximately 3 mm. It contained 6 optical fibres with a diameter of 250μm. The tube 11 coming from the extruder 15 was stretched by suitablyvarying the feeding speed along the extrusion line, using a first drivepulley 12 and a second pulley 13 for braking purposes (as the setrevolution speed was lower than that of the pulley 12). Furthermore, thecongruence between optical fibre and tube was achieved on the pulley 12by winding the tube by five turns on said wheel.

[0144] The pulley 12 and the pulley 13, both having a diameter of 250mm, were positioned with the respective centres of revolution at adistance of 350 mm from each other to form the stretching device. Saiddevice was positioned inside the cooling pool 10 (25 m long and 15 cmwide). The pulley 12 was located 2.7 m from the extruder. The coolanttemperature in the pool 10 was kept constant at 20° C. The tractionapplied to the tube in the section between the extruder and the pulley13 was less than 0.3 kg.

[0145] A 100 cm diameter drive wheel was located at the end of thecooling pool (out of the pool), at a distance of approximately 27 m fromthe extruder. The tube was wrapped on a reel with a diameter of 30 cmlocated at approximately 30 m from the extruder.

[0146] The tube production speed was set at approximately 60 m/min.

[0147] The revolution speed of the pulley 12 was set at approximately 76g/min, corresponding to a linear speed of the tube V_(A) ofapproximately 60 m/min, while the revolution speed of the pulley 13 wasset at 75 g/min, corresponding to a linear speed of the tube V₀ equal to59.5 g/min. In this case, the length of tube between the two pulleys 12and 13 was subjected to a traction of approximately 2.3 kg and wasstretched by approximately 1%.

[0148] Tube stretching was performed at a temperature of approximately60° C., the temperature at which Vestodur 3000 has a modulus ofelasticity of approximately 600 Mpa.

[0149] The drive wheel 5 was set at a revolution speed of approximately19 g/min. The length of tube between the pulley 12 and the drive wheelwas subjected to a traction of approximately 0.8 kg.

[0150] The tube was wound on the reel in random crossed turns.

[0151] At the end of the procedure the excess fibre value inside thetube was approximately 1⁰/_(oo). Two km of tube were made in this way.

[0152] The applicant, with this method, produced a total of 9 tubes,according to this invention (a total of 18 km).

[0153] As a reference, 9 samples of tube (2 km each) were prepared usingthe system described above but without the stretching device. Aspreviously described, the drive wheel 5 was located at a distance of 27m from the extruder and followed by a reel. The wheel speed was set at19 g/min, imposing a traction of approximately 0.8 kg on the tube.

[0154] The shrinkage values measured on the 9 samples made according tothe invention and the 9 test samples after one week in storage, with therelative excess fibre resulting after shrinkage, are given in Tables 1and 2. TABLE 1 Tube with stretching (excess fibre before storage: 1%)Shrinkage percentage after one week in Resulting excess Sample storagefibre 1 0.2% 1.2% 2 0.2% 1.2% 3 0.3% 1.3% 4 0.3% 1.3% 5 0.3% 1.3% 6 0.4%1.4% 7 0.4% 1.4% 8 0.5% 1.5% 9 0.5% 1.5%

[0155] TABLE 2 Tube without stretching (excess fibre before storage: 1%)Shrinkage percentage after one week in Resulting excess Sample storagefibre 10 0.6% 1.6% 12 0.6% 1.6% 13 0.8% 1.8% 14 0.8% 1.8% 15 0.8% 1.8%16 1.0% 2.0% 17 1.4% 2.4% 18 1.6% 2.6% 19 1.9% 2.9%

[0156] After analysing the data shown in Tables 1 and 2, the applicantobserved that shrinkage after one week in storage was on average 60%lower for the tubes which were subjected to a 1 % stretching, rangingfrom a mean value of approximately 1⁰/_(oo) to a mean value ofapproximately 0.4⁰/_(oo), consequently reducing the excess fibrevariations caused by shrinkage during storage.

1. Method for the production of a polymeric material tube associatedwith at least one optical fibre accommodated therein, which comprisesthe following steps: feeding said at least one optical fibre along apath to an extruder; extruding the polymeric material around saidoptical fibre to form said tube; cooling the tube to a final predefinedtemperature, at which, during said cooling the following steps areperformed: applying a first traction force to the tube containing saidoptical fibre in a first section of said extrusion line; applying asecond traction force to said tube in a second section of said extrusionline, with substantial lack of congruence between said fibre and saidtube, said second traction force being greater than said first tractionforce; applying a third traction force to said tube in a third sectionof said extrusion line, said third traction force being less than saidsecond traction force; said second traction force being such as todetermine a longitudinal shrinkage of said tube after a storage periodof one week or longer immediately after said extrusion, of at least 20%less than a similar tube that did not undergo such stretching.
 2. Methodaccording to claim 1, characterized in that said second traction forceis applied at a tube temperature at which the polymeric material has amodulus of elasticity that is less than approximately 2000 Mpa. 3.Method according to claim 2, characterized in that, at the tubetemperature at which said second traction force is applied, thepolymeric material has a modulus of elasticity that is betweenapproximately 100 Mpa and approximately 2000 Mpa.
 4. Method according toclaim 3, characterized in that, at the tube temperature at which saidsecond traction force is applied, the polymeric material has a modulusof elasticity that is between approximately 300 Mpa and approximately1500 Mpa.
 5. Method according to claim 1, characterized in that saidfinal temperature is less than approximately 40° C.
 6. Method accordingto claim 5, characterized in that said final temperature isapproximately 20° C.
 7. Method according to claim 1, characterized inthat the tube temperature during the step in which said second tractionforce is applied undergoes a limited variation.
 8. Method according toclaim 7, characterized in that the temperature variation in the lengthof tube subjected to said second traction force is approximately 10%less than the total thermal gap undergone by the tube along theextrusion line.
 9. Method according to claim 7, characterized in thatthe temperature variation in the length of tube subjected to said secondtraction force is less than approximately 20° C.
 10. Method according toclaim 7, characterized in that the temperature variation in the lengthof tube subjected to said second traction force is less thanapproximately 10° C.
 11. Method according to claim 1, characterized inthat said second traction force is predefined so as to cause astretching of at least 1% when the polymeric material of the tube ispolybutyleneterephthalate (PBT).
 12. Tube of polymeric material producedin an extrusion process and comprising at least one optical fibreaccommodated therein, characterized in that, during production, saidtube underwent stretching such that the longitudinal shrinkage of saidtube after a storage period of one week or longer immediately after saidextrusion was at least 20% less than a similar tube that did not undergostretching.
 13. Equipment for producing a tube comprising at least oneoptical fibre accommodated therein, comprising: an extruder suitable forproducing a tube of plastic material containing at least one opticalfibre; at least one cooling pool; a stretching device suitable forapplying increased pulling on a length of said tube, the temperaturevariation in said tube length being 10% less than the total thermal gapof the tube from the extruder to ambient temperature.
 14. Equipmentaccording to claim 13, characterized in that said stretching devicecomprises a driving element and a braking element, arranged between theextruder and said driving element.
 15. Equipment according to claim 14,characterized in that said driving element comprises a motor-drivenwheel or a couple of motor-driven feeding tracks.
 16. Equipmentaccording to claim 14, characterized in that said braking elementcomprises a motor-driven wheel or a couple of motor-driven feedingtracks, where the tube is fed at a speed less than the speed at which itis fed through the driving element.